Field of the Invention
The invention relates to an apparatus for identifying the position of an element that can be moved between two end positions. In particular, the element may be a lock bolt of a motor vehicle lock, e.g. of a door lock, steering wheel lock or filler cap lock. Furthermore, the invention is directed at a method of operating such an apparatus.
In the automotive technology it is often desired to be able to detect an actual locking state/unlocking state of a motor vehicle lock in order, for example, to activate or to deactivate immobilization. The lock position can be attained by checking the position of the lock bolt, which is in one of its two end positions in accordance with the respective lock state (open/locked).
However, in a great number of other cases there is also the requirement of being able to detect the respective position of an element which can be changed over selectively between two positions, in order that corresponding control measures can be implemented.
German patent DE 40 21 164 C1 essentially discloses an apparatus that is exemplified in a proximity switch. The apparatus identifies the position of an element, in particular a lock bolt of a motor vehicle lock, which can be moved between two end positions. The apparatus has a resonant circuit with a coil that can be influenced by the movable element or a part connected. A driver and evaluation circuit is connected to the coil and evaluates an oscillation parameter of the resonant circuit for the purpose of position identification. The proximity switch contains a transistor oscillator connected to two LC resonant circuits, one of which is damped in a position-dependent manner by the element to be detected.
Furthermore, DE 44 29 314 A1 discloses an inductive proximity switch having an oscillator, which feeds a transmission coil, and two sensor coils arranged in a differential connection, which are influenced to different extents by an approaching object and whose differential AC voltage is used to drive the oscillator. In order to obtain hysteresis, the tap between the two sensor coils can be connected to ground potential via a resistor and a switch, the switch being switched on or off in the event of a change in the oscillation state of the oscillator.
European patent EP 0 170 723 B1 discloses a position identification apparatus in which an element that can be moved between two or more positions is guided along at least one coil and contains alternating zones comprising ferromagnetic and electrically readily conductive zones. The coil or coils is/are fed with alternating current and is/are connected to an evaluation circuit for detecting the respective voltage drop across the coil or coils. However, the respective resultant voltage is dependent on external influencing factors, aging phenomena, guidance inaccuracies, etc., which affects the detection precision and/or the outlay on circuitry required to obtain sufficient measurement accuracy.
In quite a lot of cases, for example in the case of motor vehicle locks, it is sufficient to be able to distinguish in boolean accuracy, i.e., "digitally" between two states, that is to say between two positions of the movable element. Intermediate positions are traversed only during the positional changeover and do not have to be detected.